At present, in the art of MWD in the drilling industry such as the explorating of oil and gas, coalbed methane, shale gas trapped within shale formations, drilling, mining and so on, the formation resistivity is generally used to form stratigraphic profiles and to determine oil saturation, gas content from coal structure and mineral fractures of reservoirs, thus the formation resistivity is a primary basis for explaining and evaluating oil and gas, coal, mineral reserves in well logging. The currently known logging-while-drilling (LWD) resistivity logging technique includes LWD lateral resistivity logging, LWD electromagnetic wave propagation resistivity logging and LWD induction resistivity logging.
The principle of the LWD lateral resistivity logging primarily involves providing current by power supply electrodes, forming an electrical field in formations around a borehole, measuring the distribution of the electrical field in the formations and obtaining the formation resistivity. An apparatus for LWD lateral resistivity logging makes the drill bit as an electrode, and also may employ a loop electrode and three button electrodes approximate the drill bit to take resistivity measurement. In the case where a drill bit serves as an electrode, before mud invasion or possible damage to the borehole, the apparatus for LWD lateral resistivity logging may measure the resistivity of a thin layer of 5-10 cm. While if a three-button electrode array is employed, a high-resolution lateral resistivity measurement may be achieved, which may reduce influence of surrounding rocks, and may provide a true formation resistivity response even in brine mud or formations having a high resistivity. Besides, if a loop electrode is employed, the resistivity information within a 360° range around the borehole may be obtained.
However, the above-mentioned apparatus for LWD lateral resistivity logging has deficiencies as follows: since the technique of lateral resistivity logging belongs to the method of DC electrical logging, that is to say, it needs to have a power supply electrode to conduct DC current into a formation, and then use a measuring electrode to measure the electrical potential at certain point in the well, thus such lateral resistivity logging method may be used only when there is conductive mud in the well that provides current channels. However, during the practical drilling operation such as the oil drilling operation, sometimes, in order to obtain the information of original oil saturation in a formation, it needs to employ oil-based mud drilling, or even employ air drilling. In such cases, the DC electrical logging method cannot be used, that is, the method of LWD lateral resistivity logging is no longer applicable in said cases.
An apparatus for LWD electromagnetic wave propagation resistivity logging employs multi-coil system, the propagation frequency is 1-8 MHz, the coil system is based on the body structure of the drill collar, and the coil system is wound around the drill collar. A phase shift shallow resistivity and an attenuation deep resistivity are calculated through measuring the amplitude ratio or phase difference between different transmitting coil and receiving coil and thereafter converting the amplitude ratio or phase difference to apparent resistivity of the formation. In ideal cases, the axial resolution of the apparatus for LWD electromagnetic wave propagation resistivity logging is dependent on the interval between two receiving coils, and the measurement data at multiple depths of investigation may be used to explain the status of mud invasion. Normally, those skilled in the art will appreciate that the depth of investigation of the phase resistivity is less, while the depth of investigation of the attenuation resistivity is deeper.
A Chinese patent application publication No. CN101609169A titled “Method for improving the precision of electromagnetic wave resistivity measurement and expanding measurement range thereof” discloses that the mutual induction electromotive force which are not related to the resistivity of formation, zero signals of a circuit and base signals of an antenna system in a plot of amplitude attenuation-resistivity conversion and a plot of phase difference-resistivity conversion of the mutual induction electromotive force is eliminated by calculating the mutual induction electromotive force between a transmitting antenna and a receiving antenna, and the conversion of phase difference and amplitude attenuation to the resistivity of formation may be obtained.
Besides, the reference document titled “Basic theory of an apparatus of electromagnetic wave resistivity LWD with tilted antennas and the application for geo-steering thereof” published in Journal of China University of Petroleum calculates the response of the apparatus of electromagnetic wave resistivity LWD with tilted antennas by using a recursive matrix method for computation of the Green's function of magnetic dipole source in anisotropic horizontally stratified medium, analyzes the influence of the relative inclination of the borehole and the dip angle of the receiving coil upon the amplitude ratio and phase difference of the receiving signal, as well as the characteristics of the conventional tools and novel tools of making response to the curve horn in the direction perpendicular to the axis of the tool, whereby predicting the existence of a formation boundary earlier.
However, although the various apparatuses for LWD electromagnetic wave propagation resistivity logging at present may measure resistivity at different depths of investigation, they have deficiencies as follows.
Firstly, the signal frequency used by the apparatus for LWD electromagnetic wave propagation resistivity logging is too high, the depth of investigation is limited due to the propagation effect of the electromagnetic wave.
Secondly, the measurement result of the apparatus for LWD electromagnetic wave propagation resistivity logging will be influenced by geological factors, especially influenced by surrounding rocks, because the measurement result of the apparatus is not only limited to the formation area between receiving coils, but also related to parameters of the whole formation between the transmitting coils and receiving coils, and even the formation within a relatively small area around the transmitting coil will influence the measurement result. Therefore, the axial resolution of the apparatus for well logging depends largely on the resistivity of the formation in which the whole apparatus is located.
Thirdly, since the coil system of the apparatus for LWD electromagnetic wave propagation resistivity logging is wound on the surface of the drill collar, the manufacture process thereof is rather complicated. Moreover, the coil system may be easily abraded and thus be damaged during operating. Then, when the size of the borehole varies, it needs to rewind the coils, thus the maintenance and overhaul is rather complicated and the maintenance cost is high. Besides, similar to the apparatus for LWD lateral resistivity logging, the apparatus for LWD electromagnetic wave propagation resistivity logging is unable to work in oil-based mud.
An apparatus for LWD induction resistivity logging applies the principle of electromagnetic induction. When alternating current at constant amplitude and frequency is applied in a transmitting coil, eddy current is induced in the formation surrounding said coil, and the eddy current per se will form a secondary alternating electromagnetic field. Under the effect of the secondary alternating electromagnetic field, induced electromotive force will be generated in receiving coils. The amount of said electromotive force is associated with the conductivity of formation, and the resistivity of formation may be obtained through measuring the induced electromotive force.
The coil system of the apparatus for LWD induction resistivity logging at present employs one transmitting coil and two receiving coils, and one of said two receiving coils is the primary receiving coil while the other one is the compensatory coil. The coil system is positioned in a V-shaped groove with a reflection layer on a lateral face of the drill collar. The response of well logging is sensitive to the resistivity variation of the formation in the front area of the V-shaped groove, thus it has the characteristic of directional measurement. The apparatus for LWD induction resistivity logging is supplied with power by a battery. On the top of the battery, there is provided with a male buckle joint which may be joined to a female buckle joint on the bottom of the apparatus for LWD induction resistivity logging for transferring real-time data from the apparatus for LWD induction resistivity logging to the surface. The same survey sub may be adapted to the requirements of boreholes in different sizes.
The advantages of such an apparatus for LWD induction resistivity logging are as follows. Since the signal frequency thereof is 20 kHz, which is greatly lower than the frequency of a high-frequency apparatus, it is not easily absorbed by formations. Furthermore, the depth of investigation is deep and the range of measurement is relatively large, which may reach 0.1-1000 Ωm. Moreover, the structure of such an apparatus is simple, and one survey sub may be adapted to the requirements of boreholes in different sizes. Also, the maintenance and overhaul is relatively easy, and it is adapted to different drilling fluids.
However, such an apparatus for LWD induction resistivity logging further has deficiencies as follows. Since the apparatus employs a coil system composed of one transmitting coil and two receiving coils and having a single fixed depth of investigation, said apparatus may only provide the resistivity of formation in one radial depth of investigation, while cannot be used to explain complicated invasive profile and to separate the corrosive formations. Besides, as for a corrosive formation, mud invasion causes the resistivity thereof to vary in radial directions, since only a resistivity value in one radial depth of investigation can be obtained at a measurement point in the same depth, the apparatus for LWD induction resistivity logging cannot be used to explain the invasion condition of the formation, and the condition where the formation is invaded by mud and the reservoir permeability cannot be determined. This is disadvantageous for explanation of oil and gas reservoirs, thus it cannot be used to calculate the true formation resistivity accurately. Furthermore, as for different types of mud invasion and the resistivity in different radial depths of investigation, the characteristics of the oil-gas-water layers are different. The oil and gas may be identified according to different degrees of mud invasion influences upon multiple resistivity curves at different depths of investigation, as well as differential characteristics manifested by the oil-gas-water layers. Therefore, multi-depth resistivity measurement is significant to a LWD apparatus. However, the apparatus for LWD induction resistivity logging at present is unable to meet the requirement. Furthermore, since the designed structure for the coil system of said apparatus is fixed, each coil system may only measure the resistivity at one depth, and different coil systems have to be used to take multiple measurements in order to obtain resistivity at different depths of investigation. As a result, it is hard to carry out such a LWD induction resistivity logging manner during the practical application.
In summary, no matter which one of said LWD resistivity apparatuses is concerned, it has many deficiencies. Moreover, each of said LWD resistivity apparatuses is only dedicated to measure and calculate the radial depth of investigation, while does not mention or refer to the measurement of axial-forward depth of investigation. However, as the number of the transmitting antennas and receiving antennas of various apparatuses for LWD resistivity logging increases continuously, the transmitting frequency decreases. Therefore, the axial depth investigation becomes growingly important to the drill engineering. Consequently, the need for the method of LWD axial-forward investigation is growingly increased in the art of well drilling and logging.